Throughout the years there have been many attempts to prepare coatings and multi-layer coatings for substrates that are resistant to discoloration and/or corrosion under certain circumstances. Of particular interest are coatings and multi-layer coatings for metal substrates that are subjected to harsh environments and ultimately cleaning solutions, such as motor vehicle wheels. In particular, motor vehicle wheels are subjected to road and environmental conditions and are often cleaned with acidic cleaning solutions. Accordingly, coatings for motor vehicle wheels that are resistant to corrosion and discoloration caused by environmental factors and/or cleaning solutions are desired.
The use of coatings containing aluminum flake pigments for application to motor vehicle wheels is generally known in the art. Particularly, the aluminum flake pigment gives the coated vehicle wheel a shiny, metallic appearance that is desired by consumers. However, vehicle wheels having coatings with aluminum flake pigments, particularly those having aluminum flake pigments produced by physical vapor deposition, may be susceptible to discoloration when exposed to acidic solutions. The aluminum flakes in the coating system react with many acids including those commonly found in wheel cleaning solutions. The acids may dissolve the aluminum flakes in the thin mid-coat causing the black base coat to show through the mid-coat and clear coat thereby discoloring the wheel. The vehicle wheel may become discolored after only a fraction of the aluminum flake pigment is dissolved in the acid. The discoloration of vehicle wheels is a significant problem for automotive original equipment manufacturers (OEM) as the discolored wheels must be replaced at significant cost to the OEM.
Accordingly, a need exists for multi-layer coatings for motor vehicle wheels and other like substrates that are resistant to discoloration in acidic solutions.